This application claims the priority of Japanese Patent Application No. 2000-318841, filed Oct. 19, 2000.
1. Field of the Invention
The present invention relates to two-stroke internal combustion engines which are suited for use in portable power working machines, and in particular to two-stroke internal combustion engines which are designed to introduce air into a combustion actuating chamber prior to the introduction of air-fuel mixture, thereby making it possible to minimize the quantity of air-fuel mixture discharged from the combustion actuating chamber without being utilized for the combustion.
2. Background of Related Art
An ordinary small air-cooled two-stroke internal combustion engine that is conventionally used in portable power machines, such as chain saws or bush cutters, is constructed such that an ignition plug is disposed at the head portion of the cylinder, and an intake port, scavenging ports and an exhaust port are provided at the trunk portion of the cylinder. The intake port, the scavenging ports, and the exhaust port are opened and closed by a piston. According to this type of two-stroke internal combustion engine, one cycle of the engine is accomplished by two strokes of the piston, without undergoing a stroke which is exclusively assigned to the intake or the exhaust.
More specifically, during the ascending stroke of the piston, an air-fuel mixture is introduced from the intake port into the crankcase disposed below the piston. During the descending stroke of the piston, the air-fuel mixture is pre-compressed to produce a compressed gas mixture. This compressed gas mixture is then blown into a combustion actuating chamber disposed above the piston, thereby enabling waste combustion gas in the combustion actuating chamber to be discharged through the exhaust port. Since the scavenging of the waste combustion gas is effected by making use of the gas flow of the air-fuel mixture into the combustion actuating chamber, the unburnt air-fuel mixture is likely to be mingled with the waste combustion gas (i.e., exhaust gas), thereby increasing the quantity of air-fuel mixture to be discharged into the atmosphere without being utilized for combustion, this unburnt air-fuel mixture referred to herein as xe2x80x9cblow-by.xe2x80x9d Because of this aspect of combustion, the two-stroke internal combustion engine is not only inferior in fuel consumption as compared with a four-stroke internal combustion engine, but this engine is also disadvantageous in that a greater quantity of poisonous components such as HC (unburnt components in a fuel) and CO (incomplete combustion components in a fuel) may be included in the exhaust gas, as compared with the four-stroke engine. Therefore, even if the two-stroke engine is small in capacity, the effect of these poisonous components on environmental contamination may not be disregarded.
With a view to address these disadvantages of ordinary air-cooled two-stroke internal combustion engine, various kinds of xe2x80x9cair-preintroductionxe2x80x9d type two-stroke internal combustion engine have been proposed. Air-preintroduction type two-stroke internal combustion engines typically have an air inlet passageway for introducing external air attached to a scavenging passageway, thereby permitting air to be introduced into the combustion actuating chamber in the descending stroke of the piston, prior to the introduction of the air-fuel mixture. The air that is pre-introduced into the combustion actuation chamber forms a layer between the waste combustion gas to be discharged and the unburnt air-fuel mixture being introduced. This air layer is intended to prevent the unburnt air-fuel mixture from being mixed with the waste combustion gas, thus minimizing the quantity of blow-by. (For example, see Japanese Patent Unexamined Publications H9-125966 and H5-33657, and Japanese Patent No. 3040758).
However, these conventional air-preintroduction type two-stroke internal combustion engines include peripheral structure, such as air inlet passageways (e.g., one or more pairs of right and left scavenging passageways may be used, if the air inlet passageway is bifurcated), which are required to be separately installed outside the engine body (i.e., cylinder body and crankcase) in order introduce air into the scavenging passageway, etc. These conventional air-preintroduction type two-stroke engines have the disadvantage that this peripheral structure around the engine body, including the air-introducing passageways, becomes complicated, larger in size as a whole, and heavier in weight. In addition, the operation and assembly of this type of engine are rather troublesome, thus requiring further improvements to make it suitable for use in a portable power working machine.
Accordingly, there exists a need in the art for a two-stroke internal combustion engine which can overcome the aforementioned disadvantages associated with the conventional air-preintroduction type two-stroke internal combustion engine.
An object of the present invention is to provide an air-preintroduction type two-stroke internal combustion engine which is capable of reasonably and compactly arranging the peripheral components of engine, thereby making it possible to reduce the number of parts, to lighten the weight thereof, to save the manufacturing cost thereof, and to simplify the working and assembling thereof.
These and other objects of the invention, which will become apparent with reference to the disclosure herein, are accomplished by the two-stroke internal combustion engine according to the present invention, which comprises a cylinder body in which a piston is adapted to be fittingly inserted; a crankcase disposed below the cylinder body and defining a crank chamber; and means for forming an air-fuel mixture; and one or more pairs of Schnxc3xcrle-type scavenging passages, each allowing the combustion actuating chamber disposed above the piston to be communicated with the crankcase. The scavenging passages may be symmetrically provided on both sides of the longitudinal section which divides an exhaust port provided on a sidewall of the cylinder into two equal parts, thereby allowing air to be introduced into the scavenging passages. The air-fuel mixture may be introduced into the combustion actuation chamber from the crank chamber. During a descending stroke of the piston, the exhaust port is first opened before a scavenging port formed at a downstream end of the scavenging passageway is opened, thereby enabling air to be introduced via the scavenging passageway into the combustion actuating chamber prior to the introduction of the air-fuel mixture.
This two-stroke internal combustion engine according to the present invention is also characterized in that an air inlet passageway for introducing air into the scavenging passageways is provided inside the wall portion of the cylinder.
In a preferred embodiment, the air inlet passageway is constituted by an air inlet port, a pair of right and left linear branch passageways communicated with the air inlet port, and a communicating passageway which permits the branch passageways to be communicated with the scavenging passageway.
This communicating passageway should preferably be formed with a cap member to be attached to the cylinder.
In a preferred embodiment, the air inlet passageway is provided with a check valve. Further, a check valve may be attached to the cap member or to the branch passageways.
In further preferred embodiment, the internal combustion engine is further provided with an air-feeding passageway for feeding external air to the air inlet passageway, and with an air-fuel mixture-feeding passageway for feeding, via an air-fuel mixture-inlet port, air-fuel mixture generated by the air-fuel mixture generating means to the crank chamber, the air-feeding passageway and the air-fuel mixture-feeding passageway being arranged side by side.
In further preferred embodiment, the air-fuel mixture-inlet port to be opened and closed is disposed below the air inlet port formed in the cylinder.
In a more preferred embodiment, the crankcase is provided with the air inlet port and the air-fuel mixture-inlet port.
In another preferred embodiment, the air-fuel-generating means is a carburetor which is provided with the air-feeding passageway and the air-fuel mixture-feeding passageway, each provided with a throttle valve, and these throttle valves being interlocked with each other.
According to the preferable embodiments of two-stroke internal combustion engine of the present invention as described above, the external air is sucked via the air-feeding passageway into the air inlet port and the air inlet passageway, and at the same time, this external air is also introduced via this air inlet passageway and an air-check valve, such as a reed valve, into the scavenging passageways as well as into the crank chamber so as to be reserved therein in the ascending stroke of the piston. The air-fuel mixture supplied from the air-fuel mixture-generating means is sucked, via the air-fuel mixture-feeding passageway and the air-air-fuel mixture inlet port, into the crank chamber so as to be reserved therein.
When an air-fuel mixture inside the combustion actuating chamber disposed above the piston is exploded and burnt through the ignition thereof, the piston is caused to be pushed downward due to the generation of combustion gas. In the descending stroke of the piston, the air and the air-fuel mixture existing in the air inlet passageway, the scavenging passageways and the crank chamber are compressed by the piston, and at the same time, an exhaust port is opened first. When the piston descends further, the scavenging port provided at a downstream end of each of the scavenging passageways is subsequently opened. During this scavenging period, only the air which has been existed in the scavenging passageways and the air inlet passageway and compressed by the piston is permitted to be introduced from the scavenging ports into the combustion actuating chamber.
When the piston has descended further, the introduction of air from the scavenging ports to the combustion actuating chamber is completed, after which the air-fuel mixture that has been precompressed in the crank chamber is introduced via the scavenging passageways into the combustion actuating chamber until the scavenging period is completed.
Accordingly, since air is introduced into the combustion actuating chamber from the scavenging ports prior to the introduction of an air-fuel mixture during the descending stroke of the piston, the waste combustion gas is forced, by this action of air, to be pushed out of the exhaust port and then, discharged via a muffler into the external atmosphere without leaving a residue of waste combustion gas not only in the combustion actuating chamber but also in a portion near the inner wall of cylinder which is disposed opposite to the exhaust port.
In this case, a layer of the air that has been introduced in advance from the scavenging ports into the combustion actuating chamber is formed at an interface between the waste combustion gas and the air-fuel mixture that is introduced later from the scavenging ports into the combustion actuating chamber. Due to the existence of this air layer, the air-fuel mixture is effectively prevented from being mixed with the waste combustion gas, thereby realizing a stratified scavenging. As a result, the quantity of so-called blow-by, i.e., the quantity of air-fuel mixture to be discharged without being utilized for the combustion, can be reduced to a minimum, thus making it possible to reliably and completely ignite the air-fuel mixture, to improve the fuel consumption, and to reduce the content of poisonous components in the exhaust gas.
Further, according to the two-stroke internal combustion engine of the present invention, an air inlet passageway is provided inside the wall portion of the cylinder. Therefore, in contrast to the conventional internal combustion engine where bifurcated air inlet passageways are required to be separately installed outside the engine body (i.e., cylinder body and crankcase), it becomes possible according to this invention to reasonably and compactly arrange the peripheral components of engine, thereby making it possible to reduce the number of parts, to lighten the weight thereof, to save the manufacturing cost thereof, and to simplify the working and assembling thereof.
In this case, when a pair of the right and left branch passageways constituting a main portion of the air inlet passageway are made linear in configuration, these branch passageways can be formed by means of not only draw-molding but also drilling work, thereby making it possible to greatly improving the productivity.
Further, since the air inlet passageway is formed inside the wall portion of cylinder, the effective length of the air inlet passageway can be shortened as compared with the conventional air inlet passageway, and hence it is expected that the performance of engine such as response characteristics can be improved.
In accordance with the invention, the objects as described above have been met, and the need in the art for an engine that efficiently ignites the air-fuel mixture with a minimum of blow-by, that improves fuel consumption and reduces the content of poisonous gases, and that has a compact configuration, has been satisfied. Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of illustrative embodiments.